Carbon dioxide is utilized as a refrigerant in this invention rather than other cryogenic products such as nitrogen or oxygen because, unlike other cryogenic products that exist in only two states, liquid and gaseous, carbon dioxide exists in three forms--liquid, solid and gas. In the solid state, carbon dioxide can be fixedly located or contained to give the cooling effect required. It would be virtually impossible to contain cryogenic products in either liquid or gaseous states in a transportation vehicle.
A study conducted by the American Frozen Food Institute to find an alternative to mechanical methods of refrigeration which require extensive maintenance and labor intensive monitorings concluded that cryogenics, and especially carbon dioxide, would be a suitable alternative. The Institute created the first prototype railcar utilizing a series of CO.sub.2 tanks placed beneath the floor of the railcar. The carbon dioxide was piped to each end of the railcar and connected to two orifices at each end wall near the ceiling. It was theorized that with the addition of temperature-sensing devices contained within the product environment, injection nozzles triggered by the rising temperature of the product contained therein would be activated and the carbon dioxide in the solid form (snow) would be sprayed upon the product. Because of the direct contact of the CO.sup.2 snow to the product, the temperatures were not uniform and the products that directly contacted the solid CO.sup.2 had a very low temperature and thus certain products would become very brittle. There was also the problem of freezer burn.
In order to keep the solid carbon dioxide from coming into contact with the products contained within the shipping enclosure, the design disclosed in Fink et al, U.S. Pat. No. 4,593,536, was developed. This patent utilizes a compartment along the top region of the railcar where the carbon dioxide is deposited in the solid form (snow) and held in reserve. The compartment is able to hold enough of the solid carbon dioxide to last many days. It was therefore possible to charge the car with the amount of snow at the loading location to complete a trip without needing an additional charge, eliminating the need to carry an additional supply of liquid carbon dioxide in the railcar. Sublimation vents were provided along one side wall adjacent to the liquid manifold to allow the sublimation gasses to enter the compartment below to provide the required refrigeration. These vents in the bunker compartment also provide a pressure relief for the flash gasses which are created in the transition from liquid to snow. The cold sublimated gasses theoretically would flow out of these vents, down the side wall in fluted panels that were provided, across the floor and upwardly in the flutes of the opposite side wall. Because carbon dioxide is heavier than air, the cold gasses did not effectively flow up the opposite side wall, but instead exited down the floor areas to the vents that carried the gasses to the outside, this uneven flow left areas in the compartment not effectively cooled.
A design disclosed by Hill, U.S. Pat. No. 4,704,874, theorized that by having a center discharge manifold and providing sublimation openings in the bunker along both side walls and along the end walls, the sublimated gasses would completely envelope the product. The flow of these sublimated gasses down all walls was then channeled along the floor of the compartment. These channels were created by aluminum T-beams running the length of the rail car which then collected the cold gasses and routed it to a discharge duct to the exterior of the railcar.
Because of a basic knowledge that certain perishable products could not be allowed to be contacted by carbon dioxide vapors, and especially those products that require oxygen to exist, an alteration to the previous designs was suggested by Moe, U.S. Pat. No. 4,761,969. Moe disclosed a design that was able to operate in a plurality of modes for this purpose. This design was to create the carbon dioxide snow and store it in a flexible bladder located in the bunker compartment. The gasses produced were to be released through a bladder vent and therefore keeping these vapors channeled through the container, not coming in contact with the inner loaded compartment, therefore acting in a manner such as an ice bag. This design has yet to be produced and it is doubtful that any material which could be made into a bladder would be able to withstand the very low temperature of solid carbon dioxide.
Thomsen, U.S. Pat. No. 4,891,954, was designed to alleviate the problems that have plagued all previous approaches and to control the tremendous pressures that are created in the loading process whereby liquid carbon dioxide is being forced through a small orifice to create the solid form (snow). The pressure at which liquid carbon dioxide exists in a standard storage vessel and is subsequently transported into the manifold of the storage compartment is approximately 300 p.s.i.g. For each 100 pounds of liquid carbon dioxide that is transformed from a liquid into a solid, only 46.5% becomes the solid and the remainder becomes flash gas. It is this flash gas at the extremely high pressure that is generated during the loading cycle that must be able to exit the bunker compartment, to eliminate the possibility of exploding the bunker or destroying the integrity in the metal in the roof of the vehicle. By creating additional relief ports directly below the center manifold, it was theorized that the snow would be blown toward each wall, therefore allowing at area below the manifold to relive this flash gas pressure.
This design certainly did seem to alleviate the previous problems encountered in the design of the railcars because of their integral strength, but utilizing this same procedure in the manufacture of shipping containers failed drastically. The present invention is directed toward the alleviation of the problems of the flash gas build-up associated with all the previous inventions so that can be successfully utilized in not only railcars but also in ocean shipping containers and trailers.